"With more than four million CCTV cameras operating in the UK, photo identity cards in the pipeline and national security at the top of the political agency, it has never been more important to understand these limitations."

Government plans for national identity cards by 2009, depend on police being able to make accurate comparisons of the photograph with the holder.

Yet photos of different people can be more similar than photographs of the same person, he said.

"It is fundamentally flawed," he said. "I don't think my passport photo which was taken last year, and is valid for another nine years, looks like me.

"You can tell it is a white male in his 30s and you can tell it's not Boris Johnson. But as far as singling out an invidual with a high degree of certainty? I point to the case of Jean Charles de Menezes who was shot by police in 2005.

"How confident do you have to be before you decide to shoot somebody.?"

Plans to use computer face recognition software to identify criminals captured on CCTV are similarly flawed. Although the technique is often shown on TV shows like Spooks and 24, in practice computers find it too difficult.

In order to get around the problem of face recognition – which also affects passports and proof of age cards used in pubs and off-licences – Dr Jenkins has devised a computer programme that scans a dozen photographs of someone to create an "average face".

People find these composite images far easier to recognise than a conventional photograph, he said.

Dr Jenkins scans around a dozen photos of a person into a computer. The computer then removes any confusing information such as shadows, expressions and clothing, and just keeps common features found in every picture.

Although the resulting pictures look peculiar, they contain the "essence" of the person, Dr Jenkins said.

In tests, volunteers were much better at identifying celebrities such as Brad Pitt, Nic Cage and Jack Nicolson from these computer generated "average photos" than they were from a single conventional picture.

Computers also find it easier to make accurate matches, he told the British Associaiton Science festival in York yesterday.

"They look almost better than the real thing,” he said. “People are faster, better and more accurate."

He added: "The question that rises is what is the best image to have on a passport of ID card. On the basis of this evidence, I would say it's better to have an average rather than a photo."

Although creating an average photograph of a face is relatively cheap, and can be done on a PC, the technique is complicated, he conceded.

It involves scanning in around a dozen different photographs taken in different settings – and could prove too difficult to introduce for passports and ID cards.

Psychologists have repeatedly shown how bad people are at checking ID cards. In a 1997 study, supermarket check-out staff were asked to check experimental credit cards carrying photographs of the owner in a real-life situation.

Even thought they knew they were taking part in an experiment, around half the fraudulent cards were accepted and 10 per cent of valid cards were rejected.